.alpha.-Amylases are enzymes which hydrolyze only .alpha.-1,4-glucosidic linkages in molecules of starchy polysaccharides such as starches, amylose and amylopectin. Since the discovery of an .alpha.-amylase from a malt extract by Payen and Persoz in 1833, they have been obtained as crystalline samples or electrophoretically uniform samples from various living organisms, for example, bacteria belonging mainly to the genus Bacillus, such as Bacillus subtilis Marburg, Bacillus subtilis natto, Bacillus amyloliquefaciens, Bacillus licheniformis, Bacillus cereus, Bacillus circulans, Bacillus macerans, Pseudomonas stutzeri and Klebsiella aerogenes; Actinomycetes such as Streptomyces griseus; molds belonging mainly to the genus Aspergillus such as Aspergillus oryzae and Aspergillus niger; seeds of gramineous and leguminous plants; and digestive glands of animals such as human and swine.
.alpha.-Amylases have been used widely for many years, for example, for the saccharification of grains and potatoes in the brewing industry, as a desizing agent in the fiber industry, as a high-efficacy digestive in the pharmaceutical industry and for the production of thick malt syrup in the food industry.
The present inventors previously found that the incorporation of such an .alpha.-amylase in a dish washing detergent or in a laundry detergent together with a debranching enzyme makes it possible to prepare a detergent having significantly improved detergency against starch stains and already filed a patent application thereon (Japanese Patent Laid-Open No. 132192/1990).
Most of .alpha.-amylases which have heretofore been found in the natural world exhibit maximum and stable enzymatic activity in a neutral to acidic range, in other words, are classified as neutral to acidic .alpha.-amylases, leading to the drawback that their activity is deteriorated in a surfactant-containing solution whose pH is on the alkaline side. On the other hand, .alpha.-amylases exhibiting maximum activity in the alkaline range or having alkali resistance, in other words, alkaline .alpha.-amylases and alkali-resistant .alpha.-amylases are free of the above drawback and are useful as components for detergents. Known examples of such alkaline amylases and alkali-resistant .alpha.-amylases are limited only to the enzyme produced by the strain Bacillus sp. A-40-2 [Agric. Biol. Chem., 35, 1783 (1971)], the enzyme produced by the strain Bacillus sp. NRRL B-3881 [J. Bacteriol., 110, 992 (1972)], the enzyme produced by Streptomyces sp. KSM-9 (Japanese Patent Laid-Open No. 209588/1986, the enzyme produced by the strain Bacillus sp. H-167 (Japanese Patent Laid-Open No. 208278/1987), the enzyme produced by the strain Bacillus alcalothermophilus A3-8 (Japanese Patent Laid-Open No. 49584/1990) and the enzyme produced by the strain Natrococcus sp. Ah-36 (Japanese Patent Laid-Open No. 211369/1992). Incidentally, the term "alkaline .alpha.-amylase" as used herein means an .alpha.-amylase whose optimum pH is in an alkaline range, while the term "alkali-resistant .alpha.-amylase" means an .alpha.-amylase which has an optimum pH in a neutral to acidic range, but even in an alkaline range, can still exhibit activity comparable with that at an optimum pH and retains stability. The term "neutral" as used herein means a pH range of from 6 to 8 and "alkaline" means a pH range higher than the above range.
To the best of the present inventors's knowledge, these alkaline and alkali-resistant enzymes belong to so-called saccharifying .alpha.-amylases which degrade starches or starchy polysaccharides to glucose, maltose or maltotrise. They are hence suited as enzymes for the production of sugar but not as enzymes for detergents. Paying attention to so-called liquefying .alpha.-amylases which are resistant to surfactants and in addition, degrade starches or starchy polysaccharides at high random, the present inventors have proceeded with an investigation. As a result, it has been found that conventionally known alkaline .alpha.-amylases are only those showing the characteristics of the saccharifying type and there is absolutely no report on liquefying alkaline .alpha.-amylases.
Furthermore, to obtain the conventional alkaline .alpha.-amylases, purification of several steps is required. This purification is not satisfactory as a purification method for enzymes on an industrial scale. With a view toward obtaining a purified enzyme easily by using the characteristics of enzymes, the present inventors paid attention to an enzymatic protein having a high surface charge. As a result, it was found that all the alkaline .alpha.-amylases known to date have an isoelectric point of about 3.0-8.0 and there is utterly no report on alkaline .alpha.-amylases having an isoelectric point exceeding 8.5 as in the present invention,
An object of the present invention is therefore to provide an .alpha.-amylase which has resistance to surfactants, has an optimum pH in an alkaline range, is of a liquefying type, that is, degrades starches or starchy polysaccharides at high random and has a high isoelectric point; and also a detergent composition containing the .alpha.-amylase.